1. Field of the Invention
The present invention relates to a process for producing methacrylic acid by the vapor phase catalytic oxidation of methacrolein with molecular oxygen or a molecular oxygen-containing gas. More particularly, it relates to a process for producing methacrylic acid in high yield by the vapor phase catalytic oxidation of methacrolein over a multi-element complex oxide catalyst of Mo, V, P, Ce and Cu. If desired, at least one element selected from the group of Mn, Fe, Co, Sn and Te can also be incorporated in the catalyst. The catalyst containing the optional metal component can also be optionally reduced with a reducing organic material.
2. Description of the Prior Art
Methacrylic acid esters such as methyl methacrylate which are commonly used as starting materials for transparent plastics have been produced by the acetone-cyanohydrin process which employs acetone and hydrogen cyanide. However, the conventional process has several disadvantages in that large amounts of by-products are formed and an expensive procedure is required for the recovery of cyanide and sulfuric acid and hydrogen cyanide, which are the highly toxic starting materials. Moreover, the starting materials are not easily available. Because of these drawbacks, it is highly desirable to develop a new economical and rational process for the industrial synthesis of methacrylic acid esters.
The development of a process for producing methacrylic acid esters from isobutylene as the starting material has been greatly anticipated and desired. A process involves the vapor phase oxidation of isobutylene in the spent B-B fractions to produce methacrolein which in turn is oxidized to methacrylic acid. The acid is finally esterified to a methacrylic acid ester.
Another process for producing acrylic acid esters by the direct oxidation of propylene as the starting material has been conducted in many large plants. However, a similar process for the production of analogous compounds, i.e., methacrylic acid esters, has not been developed on an industrial scale. One of the main reasons for the difficulty encountered in developing a satisfactory procedure is the difficulty of oxidizing methacrolein to methacrylic acid. In practice, even though Mo-V type (and Mo-V-W type) catalysts, which exhibit excellent catalytic activity in the production of acrylic acid from acrolein, have been used for the catalytic oxidation of methacrolein to methacrylic acid, the desired results have not been attained. The single flow yield of methacrylic acid from methacrolein has only been as high as about 10 to 20%. Furthermore, when the Mo-P type catalysts which have been proposed for use in various processes have been used as catalysts for producing methacrylic acid by the vapor phase oxidation of methacrolein, low single flow yields of methacrylic acid have been obtained and the activity of the catalysts has been too low to achieve the desired results on an industrial scale.
With regard to the process for producing methacrylic acid by the vapor phase oxidation of methacrolein with a molecular oxygen or a molecular oxygen-containing gas, many patents have issued which show various Mo-P type catalysts such as Japanese Patent Publication No. 24288/1975, which more specifically shows a catalyst system of Mo, P and Tl containing at least one element from the group of Si, Cr, Al and Ti; Japanese Unexamined Patent Publication No. 96552/1975 which shows a catalyst system of Mo, P and V containing at least one element from the group of Na, K, Rb and Cs; and Japanese Unexamined Patent Publication No. 123619/1975 which shows a catalyst system of Mo, P, V containing at least one element from the group of K, Rb, Cs and Tl, and if desired at least one element from the group of Sr, Zn, Cd, Nb, B, Pb, Bi and W. However, even though an alkali metal, an alkaline earth metal or another metal component has been added as a promoter to the Mo-P type catalysts, as proposed in the prior art, it has been difficult to attain satisfactory results for an industrial scale operation because the promotional effects attributable to the presence of the additional metal component are insufficient. Moreover, the stability of the lifetime characteristics of the catalyst in use is an important and an indispensible industrial factor in addition to catalytic activity and selectivity factors.
Generally, it is known that Mo-P type catalysts possess inferior heat stability at high temperatures which is the cause of substantial losses in catalytic activities, as disclosed in many patents such as Japanese Patent Publication No. 27526/1965 and Japanese Unexamined Patent Publication No. 33082/1972. However, the disadvantages of the Mo-P type catalysts cannot be overcome by the simple addition of an alkali metal or other component to the catalyst because of the substantial characteristics of the Mo-P type catalysts. The reason for this may be that the dehydrating condensation of phosphoric acid and the growth of crystals of molybdenum trioxide occur simultaneously in the catalyst at high temperatures, especially higher than 340.degree. C., which results in a sudden decrease of the surface area and the development of a fine surface pore structure.
An improved catalyst containing Mo, V, P, and Ce which possesses substantially improved catalytic activity has been developed. However, it would be desirable to have a catalyst which possesses high activity at lower temperature (especially about 270.degree. C. to 280.degree. C.).
A study has been conducted and it has been found that Mo-V-P-Ce-Cu type catalysts which contain at least one element of the group of Mn, Fe, Co, Sn and Te have very high catalytic activity at relatively low temperature and the yield of product is further improved by reduction of the catalyst by treating the catalyst with a reducing organic material such as a dibasic carboxylic acid or an oxycarboxylic acid.
Another Mo-P catalyst is known for the conversion of methacrolein to methacrylic acid as disclosed in U.S. Pat. No. 3,875,220. The catalyst in addition to Mo and P optionally contains one or more elements of the group of bismuth, arsenic, boron, cerium, chromium, silver, iron, tungsten, nickel, niobium, lead, manganese, thallium, tellurium, tin or copper. However, cerium and copper are not essential components of the catalyst, while these elements are essential components of the present catalyst. Moreover, it has been found experimentally that the improved results of the present invention can only be obtained with the present catalyst having the components specified.